The Pope did not explicitly call for a boycott of the vote, which needs a 50% turnout to remain valid.

But he told a conference of Italian bishops, which had urged a boycott: "I am close to you in word and prayer."

The bishops have taken a high profile campaigning role ahead of the vote.

The Pope told them that Italians must defend "the sacredness of human life and the promotion of the role of the family in society".

The fertility debate has become Italy's most divisive social issue in decades.

Debates over divorce and abortion polarised popular opinion in Italy during the 1970s.

Millions of signatures

The German-born Pope, who oversaw Catholic doctrine under his predecessor, John Paul II, has already vowed to maintain a tough line on moral issues such as euthanasia and abortion.

Italy's current laws ban embryo research and restrict the numbers of eggs that can be fertilised during each attempt at insemination.

Egg and sperm donation is also banned, while fertilised eggs are currently afforded full rights under Italian law.

Opponents of the laws, passed only last year, forced a referendum after collecting almost four million signatures of protest.

Daniele Capezzone, head of Italy's Radical Party, which is campaigning for a "Yes" vote, called the Pope's intervention "an unprecedented offensive" by the Vatican.

The head of the Italian bishops' conference, Cardinal Camillo Ruini, has consistently taken a leading role in the campaign for a boycott, backing the current laws despite saying they "do not correspond to the ethics of the Church".